1. Field of the Invention
The present invention relates to a vibration gyro that detects an angular velocity using a Coriolis force.
2. Description of the Related Art
For example, a digital video camera or a digital still camera detects an angular velocity using a vibration gyro in order to correct a hand motion (see, for example, see Japanese Unexamined Patent Application Publication No. 2000-205861).
FIG. 1 is a drawing showing an example of a related-art vibration gyro.
A vibration gyro 101 includes a piezoelectric vibrator 122, an addition circuit 134, an oscillation circuit 136, a detection circuit 142, and a phase inversion circuit 138.
The piezoelectric vibrator 122 is a pillar-shaped vibrator. FIG. 1 shows a section of the pillar. The piezoelectric vibrator 122 includes a drive electrode 126, detection electrodes 128A and 128B, an intermediate electrode 130, and piezoelectric bodies 124A and 124B. The intermediate electrode 130 is interposed between the piezoelectric bodies 124A and 124B. The piezoelectric bodies 124A and 124B are polarized in the directions of arrow P in FIG. 1. In the piezoelectric vibrator 122, the piezoelectric bodies 124A and 124B expand or contract in directions perpendicular to the surface of FIG. 1 on the basis of a sinusoidal drive voltage applied to the drive electrode 126 by the oscillation circuit 136. Since the piezoelectric bodies 124A and 124B expand or contract in opposite directions, the entire piezoelectric vibrator 122 bends and vibrates vertically. When an angular velocity is applied to the piezoelectric vibrator 122 due to a hand motion or other external force, the piezoelectric vibrator 122 becomes deformed due to a Coriolis force in a direction perpendicular to the vibration direction (e.g., vertically). In the piezoelectric vibrator 122, a potential difference corresponding to this deformation occurs between the detection electrodes 128A and 128B.
The addition circuit 134 adds and averages detection voltages of the detection electrodes 128A and 128B and outputs the resultant voltage. The oscillation circuit 136 outputs a drive voltage. The detection circuit 142 detects an electromotive voltage due to a piezoelectric effect from the potential difference between the detection voltages of the detection electrodes 128A and 128B. The phase inversion circuit 138 inverts the phase of a voltage output from the addition circuit 134 and outputs the resultant voltage to the detection electrodes 128A and 128B via detection resistances.
In the above-described configuration of the vibration gyro, the detection voltages detected by the detection electrodes 128A and 128B are voltages obtained by dividing the drive voltage using a voltage divider circuit including a drive resonant resistance of the piezoelectric vibrator 122 and the detection resistances 140A and 140B. As shown in the following formula, the drive voltage and detection voltages are proportional to each other:
  V  =                    V        drv            ×              R        k                            2        ×        Z            +              R        k            where V [Vpp] is the amplitude of the detection voltage, Vdrv [Vpp] is the amplitude of the drive voltage, Rk [Ω] is the detection resistance (including two parallel resistances) and Z [Ω] is the drive resonant resistance of the piezoelectric vibration unit.
This proportionality factor is a positive value less than 1. Therefore, in the vibration gyro 101, a both-end voltage (Vdrv−V) of the piezoelectric vibrator is increased as the drive voltage Vdrv or detection voltage V is increased. Conversely, the both-end voltage (Vdrv−V) of the piezoelectric vibrator is reduced as the drive voltage Vdrv or detection voltage V is reduced. Therefore, in order to increase the mechanical drive amplitude or sensitivity of the vibration gyro 101, both the drive voltage Vdrv and the detection voltage V must be increased.
Generally, when a piezoelectric vibrator is separately excited by an oscillation circuit in a vibration gyro, the oscillation frequency may deviate from the resonance point of the piezoelectric vibrator. For this reason, matching the oscillation frequency with the resonance point of the piezoelectric vibrator by providing an automatic gain control (AGC) circuit and a phase-shift circuit and causing the piezoelectric vibrator to excite itself using these circuits as been considered. In this case, the AGC circuit outputs a drive voltage subjected to automatic gain control so that an output voltage of the addition circuit has a given amplitude. The phase-shift circuit controls the phase of the drive voltage so that the piezoelectric vibrator excites itself.
In a configuration including the AGC circuit, if the amplitude of the drive voltage is too large, the drive voltage deviates from the dynamic range of the AGC circuit and then is clipped. Thus, a problem such as an oscillation stop occurs. For this reason, the drive voltage must be set so that it falls within the dynamic range of the AGC circuit. In addition, recently, vibration gyros have been required to reduce an input power supply voltage provided from the set that includes the gyros. Therefore, in order to confine the drive voltage within the dynamic rage of the AGC circuit, the drive voltage must be further reduced.
For example, in order to reduce the input power supply voltage to Vcc=2.6 V, it is necessary to reduce the amplitude of the drive voltage to 2.0 Vpp or less while considering the transistor saturation voltage of a drive amplifier of the AGC circuit. If the amplitude of the drive voltage is reduced, the both-end voltage or the mechanical vibration amplitude of the piezoelectric vibrator is also reduced. As a result, it is difficult to obtain high-sensitivity gyro characteristics.